A typical cigarette comprises tobacco wrapped with cigarette paper and forms together with it a typically cylindrical tobacco rod. Attached to the tobacco rod is the filter that typically consists of cellulose acetate fibers. The filter and the tobacco rod are wrapped with the tipping paper. The tipping paper connects the filter to the tobacco rod. Aside from its function to wrap the tobacco, the cigarette paper must, inter alia, provide the cigarette with a pleasant outward appearance in the burnt and unburnt state and influence the smoldering speed of the cigarette. It also serves essentially to control the composition of the smoke, in particular the tar, nicotine and carbon monoxide content.
During smoking, the smoker generates a low pressure at the mouth end of the smoldering cigarette, by means of which air is on the one hand drawn through the glowing cone at the tip of the cigarette, but on the other hand also flows through the air-permeable cigarette paper into the tobacco rod and thus dilutes the smoke. The air flow flowing through the air permeable cigarette paper into the tobacco rod during smoking is called rod ventilation.
In the case in which the tipping paper is perforated, air flows through the tipping paper into the filter and also dilutes the smoke. This air flow is called filter ventilation.
Total dilution of the smoke can be broken down into the rod ventilation and the filter ventilation. During smoking and during free smoldering the tobacco rod is burnt, so that its length decreases. In this manner, the area of the cigarette paper available for rod ventilation is also reduced so that less and less air can flow through the cigarette paper into the tobacco rod and hence the rod ventilation decreases from puff to puff. To the same extent, the smoke is diluted less puff by puff and the concentration of the aerosols and gases that form the smoke increases in the smoke flowing out of the mouth end of the cigarette. Additionally, the tobacco rod has a certain filtration effect on the smoke that also decreases steadily with decreasing length of the tobacco rod. The smoker therefore gets the impression that the cigarette gets “stronger” from puff to puff.
This impression is undesirable and various measures are known in the prior art to mitigate it. As an example, the cigarette paper can be perforated, wherein the part of the cigarette paper lying close to the filter is perforated more strongly and has therefore higher air permeability than the remaining cigarette paper. In this manner, the rod ventilation does not decrease as strongly as for a cigarette paper with approximately constant air permeability along the tobacco rod. This procedure sometimes has the disadvantage that such cigarettes are hard to light, because a lot of air is flowing through the more strongly perforated part of the cigarette paper and the air flow through the tip of the cigarette is too small to start the smoldering process during lighting.
An alternative measure consists in coating the cigarette paper on the part further remote from the filter such that the air permeability of the coated areas is reduced and thereby areas of the tobacco rod are consumed first that contribute less to rod ventilation, as proposed in U.S. Pat. No. 3,911,932. Often, however, this measure will lead to a disproportionate increase of the carbon monoxide content in the smoke.
Furthermore in U.S. Pat. No. 3,667,479 it is proposed to coat the cigarette paper in parts of its area with strong oxidizing agents. During smoking the paper will be quickly thermally degraded in the coated areas and openings are created through which air can flow and dilute the smoke. The size or number of the areas increases in the direction towards the mouth end. The disadvantage is that such cigarettes provide a very bad ash appearance. After smoking, the tobacco ash of a cigarette is expected to remain as a white, cohesive column. Black stains and protruding ash particles or holes are not desirable. But coating parts of the area means that exactly such undesired holes are created.
Finally, for example, in U.S. Pat. No. 3,805,799 multi-layer coatings of substances that are degradable and non-degradable by the smoke are proposed. Such solutions, however, are not prevalent.